Shifts in fungal biomass and activities of hydrolase and oxidative enzymes explain different responses of litter decomposition to nitrogen addition

Exogenous nitrogen (N) input is a key factor affecting litter decomposition. However, we have limited understanding on how anthropogenic N deposition affects the kinetics and thermodynamics of enzymes involved in litter decomposition. To understand how N enrichment influences litter decomposition, we conducted a field N-addition experiment with Castanopsis chinensis (CC) and Schima superba (SS) leaf litter. We examined microbial community composition, activities of hydrolases and oxidative enzymes, and hydrolase kinetics and thermodynamics. The litter mass remaining after 18 months of decomposition in N-addition plots was 1.9-2.3 and 1-1.3 times higher than control for CC and SS, respectively. During the early stage of litter decomposition, N addition increased hydrolase activities involved in carbon (C) and N mineralization for both litter types. N addition slowed CC litter mass loss, and reduced the activities of lignolytic enzymes and catalytic efficiency (V-max/K-m) of hydrolases in the later stage (9-18 months) of decomposition. N addition had minimal effect on the activation energy (Ea) of enzymes. Our study identifies how enzyme kinetics regulate litter decomposition under N fertilization, and lignin enrichment as decomposition progresses due to microbial N mining effect limits the accessibility of lignin-encrusted structural carbohydrates to hydrolases, which subsequently decreases the energy source of the entire microbial community for enzyme production.